This invention relates to a method for fabricating a bipolar semiconductor device, and more particularly to a method for fabricating a bipolar semiconductor device with the emitter and base contact positioned in self-alignment and possessing shallow emitter-base junction suited to high speed operation.
As one of the causes of characteristic fluctuations in the fabrication of integration of bipolar device, for example, the variation of the base resistance due to discrepancy in positioning of the emitter region and base-contact region is known. Due to the change in base resistance, in terms of DC, fluctuations of the base-emitter voltage occur, and pair performances of various analog devices is worsened, and in terms of AC, fluctuations of high speed switching speed are increased. To solve these problems, conventionally, for example as disclosed in the Japanese Pat. No. 1365508 (issued Feb. 26, 1987), after forming the base region of a vertical NPN transistor, emitter contact holes and base contact hole were formed in the insulation film on said base region, and the emitter region was formed from said emitter contact hole in order to form the base contact and emitter region by self-alignment to reduce the fluctuations of the base resistance, thereby stabilizing the characteristic changes. According to this technique, the base contact holes and the emitter region can be formed in self-alignment, but it is necessary to form an intrinsic base region before forming the emitter contact hole and base contact holes, and therefore it is frequently difficult to form the intrinsic base in a very shallow depth so as to be suited to high speed operation. Moreover, when anisotropic dry etching is employed for forming fine contact holes, etching damage or impurity ion contamination is likely to occur in the prospective part for forming the emitter, which often led to deterioration of device characteristics. Incidentally, without directly dry-etching on the prospective part for forming the emitter, a method of forming an intrinsic base region after forming emitter contact hole was disclosed, for example, in the 1984 International Electron Device Meeting Digest of Technical Papers (pp. 753-756). What is characteristic of this technique is the forming of an oxidation-resistant mask material pattern made of a silicon nitride film or the like on the intended part for forming the emitter, and, using it as the mask of diffusion, form an external base region around the prospective part for forming the emitter by the boron thermal diffusion with boron silicate glass (BSG), then, using this oxidation-resistant mask material pattern as the mask, oxidize selectively, thereby forming the emitter contact hole. Furthermore, by ion implantation of boron or the like, an intrinsic base is formed, and by ion implantation of arsenic or the like, an emitter is formed. In this method, however, since the diffusion length in the lateral direction from the external base to the intrinsic base by BSG is great, the dimensional change of the intrinsic base is great and since the emitter region is to be deeper than the damage layer part due to boron ion implantation for intrinsic base, it is necessary to form the emitter deeper. Therefore the base structure is not suited to high speed operation. Additionally, in this technique, the base contact holes are formed by a mask alignment method, and a self-aligned method for forming the base contact holes and emitter contact hole is not disclosed at all.
Furthermore, it is desired to open the contact holes of emitter, base and collector or the windows of diffusions simultaneously from the viewpoint of simplification of the process and the enhancement of yield.
Furthermore, it was difficult to achieve a fabricating method of forming the base contact and emitter region of highly integrated bipolar device finely in self-alignment, and preventing abnormal diffusions in the intrinsic base region, various damage layers to cause electric leaks, and occurrence of contamination.